Nobody really knows the exact cause of the Permian/Triassic extinction, but it has been discussed by many experts. Generally, it is agreed that many things caused it.[2][3] The Triassic had several more extinction events, whose causes are also obscure. The most significant of these took place at the end of the Triassic, which was one of the 'big five' Phanerozoicmarine extinctions.[4][5]

During the Triassic, almost all the Earth's land mass was concentrated into a single supercontinent centered more or less on the equator, called Pangaea ("all the land"). All the deep- ocean sediments, or the things that settled at the bottom of the ocean, have disappeared because oceanic plates moved sideways and downward until they finally were on top of or under each other. The supercontinent Pangaea was rifting during the Triassic–especially late in the period–but had not yet separated. Because of the limited shoreline of one super-continental mass, Triassic marine deposits are globally relatively rare, despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west.

The first part of the Triassic had much less variety than the Permian, and showed signs of a deteriorated environment. This situation lasted for about 5 million years, then steadily improved. Into the vacant ecological niches new forms evolved, replacing old with new. This rapid adaptive radiation happened to the reptiles on land and the fish in the seas, and to a number of other groups, such as the insects. Diverse communities with complex ecology took 30 million years to reestablish.[6]

The group of bony fish known as the teleosts first appeared in the Triassic. The group now includes over 80% of all fish and 95% of all bony fish. The only other common group of fish are the Chondrichthyes, the sharks and rays, and the rays are also a Triassic 'invention'.

Many reptile goups originated in the Triassic, possibly because so many niches were vacant at the time. Some of these new groups were caught in the end-Triassic extinction, but those that survived ruled the Earth for the rest of the Mesozoic.[7] examples are:

This was an extaordinary turn-around, for in the middle Triassic the ancestors of mammals, the Synapsids, were still dominant. All the same, many reptile groups went extinct in the Triassic:[7]p41

Crocodilian Archosaurs:

Ornithosuchids

Aetosaurs

Rauisuchians

Other Archosaurs:

Euparkeria

Erythrosuchids

Proterosuchids

Proto-Archosaurs

Rhyncosaurs

Trilophosaurs

Prolacertiforms

Anapsids

Protocolophonids

And, amongst the Synapsids (formerly Mammal-like reptiles), the End-Triassic extinction saw the last of the Dicynodonts. Thus the Triassic is seen to be a period of great turnover in tetrapods: many new and important groups, but many extinctions. The fossil record of the Triassic is the poorest in the Mesozoic, which makes it difficult to explain these events.